[GHC] #13054: Generating unique names with template haskell

2 Jan 2017

      #13054: Generating unique names with template haskell
-------------------------------------+-------------------------------------
           Reporter:  tim-m89        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I need to use template haskell to generate a few instances where those
 instances each use a foreign function obtained from a particular
 {{{FunPtr}}}.

 As far as I'm aware, the only clean way to do this requires top level
 declarations of the form:
 {{{foreign import ccall "dynamic" makeFun1 :: FunPtr Foo -> Foo}}}

 If there was either:

 * Another way to do this without requiring top level named declarations.
 * The possibility to use these within a where clause (template haskell
 would suggest so given that it lets you use {{{Dec}}} within a
 {{{where}}}).

 Then I'd be fine probably be fine without names.

 I've tried using {{{newName}}} as it apparently generates fresh names but
 I cannot get it work for me.

 I've put a copy of the test case [https://gitlab.com/tim-m89/hs-th-name-
 issue here]. I compile with {{{stack build --ghc-options="-ddump-splices
 -ddump-to-file"}}} and the error is:

 {{{
 th-name-issue/app/Main.hs:7:12: error:
     Multiple declarations of ‘makeFun’
     Declared at: app/Main.hs:7:12
                  app/Main.hs:7:12
 }}}

 Then I run {{{gvim .stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/th-
 name-issue-exe/th-name-issue-exe-tmp/app/Main.dump-splices}}}

 And can see:

 {{{
 app/Main.hs:7:12-25: Splicing declarations
     template-haskell-2.11.0.0:Language.Haskell.TH.Quote.quoteDec
       importGen " stuff here "
   ======>
     foreign import ccall safe "dynamic" makeFun_a3pc
       :: GHC.Ptr.FunPtr (Foreign.C.String.CString -> IO ())
          -> Foreign.C.String.CString -> IO ()
     foreign import ccall safe "dynamic" makeFun_a3pd
       :: GHC.Ptr.FunPtr (Foreign.C.String.CString -> IO ())
          -> Foreign.C.String.CString -> IO ()
 }}}

 Which suggests that I did manage to get unique names after all, so I don't
 know why it didn't compile??

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13054
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